It is well known that the direction of magnetization of a magnetic material can be reversed or switched by impressing an external magnetic field of opposite polarity on the material. Typically, magnetic material may be placed upon a nonmagnetic substrate and then divided or separated into a grid-like pattern to create magnetic post elements. When electric conductors are placed within the separations which form the grid-like pattern, electric currents may be passed through these conductors for generating a magnetic flux about each conductor. When this flux is concentrated at the point of coincidence of two conductors, the concentrated flux may be used to reverse the magnetization of the post element.
If the magnetic post element and its nonmagnetic substrate are structured from transparent materials and the post element material exhibits the Faraday effect, it is possible to create a magneto-optic display by passing polarized light through the transparent post element and then through an analyzer.
Examples of prior art devices for switching the direction of magnetization of magnetic post elements upon a nonmagnetic substrate to create a magnetic bubble domain memory device may be found in U.S. Pat. No. 3,715,736, issued Feb. 6, 1973 and in U.S. Pat. No. 4,114,191, issued Sept. 12, 1978. Yet, another prior art patent which uses an external field and coincident current selection of post elements in magneto-optic display may be found in British Pat. No. 1,180,334, published Apr. 5, 1968.
In order to establish a magnetic flux large enough to switch the magnetic post elements, it is often necessary to exceed a switching field value of sixteen thousand (16,000) Oersteds (Oe). A co-pending patent application entitled "Altering The Switching Threshold Of A Magnetic Material," filed Nov. 12, 1981, Ser. No. 320,819, which is assigned to the assignee of the present invention, discloses a method and device for reducing the required switching fields by the formation by implantation of a low anisotropy material within a high anisotropy material which forms the post element.
The first region of relatively low anisotropy characteristics, when compared to the high anisotropy characteristics of the post element, is utilized to initiate the switching of the direction of magnetization of the post element. When the magnetic field is reversed, the first region of low anisotropy material will reverse its magnetization, thereby creating a domain wall which will move through the low anistropy material to the interface between the first region of low anisotropy magnetic material and the high anisotropy magnetic material. The propagation of the domain wall then continues beyond the interface between low and high anisotropy material through the high anisotropy material. The completion of magnetic field reversal within a post element is thus accomplished by domain wall movement and not by coherent rotation of the magnetization. In this way, the full post element may be switched by a relatively low magnetic field, less than 200 Oe.